[time-nuts] Re Danjon Astrolabe
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Tue Sep 26 19:06:46 EDT 2006
Poul-Henning Kamp wrote:
> In message <451976CA.1000602 at pacific.net>, Brooke Clarke writes:
>> A telescope and PIN diode might be the best way to get precise timings.
>> If a fixed scope is used with a reticule where the star crosses twice or
>> more then a plot of brightness vs. time will have a flat top pulse for
>> each line crossed. The leading and trailing edges of these pulses can
>> be measured to hopefully milliseconds.
> If you want to seriously play with this, study the Hipparcos
> astrometry missions telescope design carefully.
> You can probably find the docs if you start here:
> Historical tidbit: The Hipparcos main catalogue contains only 100k
> stars, but Erik Høg (Copenhagen University) proposed some minor
> changes to the navigation sensors and have used the resulting data
> to make first the Tycho-1 catalogue of 1M stars at slightly lower
> precision, than the Hipparcos catalogue, and then reprocessing the
> data he subsequently generated the Tycho-2 with 2.5M stars.
> time-nuts mailing list
> time-nuts at febo.com
One of the most important aspects of an astrometric telescope is
stability of its focal length.
Your options are :
1) A refractor employing conventional glasses definitely not an
apochromat using ED glasses as the temperature coefficient of the focal
length of such apochromats is quite large.
2) A reflector using a single powered mirror with a field corrector such
as a Schmidt, Maksutov, or even a a hyperboloidal primary with a small
zero power refractive field corrector.
3) A Schupman Medial (uses a single glass objective together with a
Mangin mirror and a field lens or mirror to produce a instrument with
extremely small chromatic aberrations). However the filed of view may be
too small. The 1m Swedish solar coronagraph uses a schupman medial design.
The equivalent focal length of mirror telescope designs using multiple
powered mirrors such as Cassegrains, Gergorians and their variants
depends critically on the spacing between the mirrors. Whilst heroic
measures such as using invar or carbon fibre spacers can help its better
to use a design which doesn't have this inherent instability. If you are
use a red or yello filter an apochromat isn't necessary.
For observations during the daytime its also important to use a sunshade
to avoid solar heating of the lens or mirror cell.
This avoids heating induced spherical aberrration due to radial
temperature gradients in the lens or mirror.
It is possible to design a refractor objective with low sensitivity to
radial temperature gradients, however special glasses are required for
the best performance.
Baffling to minimise stray light is also critical, this is simpler with
refractors and single powered mirror designs although using a Lyot stop
can be effective even with a Cassegrain or Gregorian.
The design of the HST fine guidance sensors may also be of interest.
Atmospheric seeing will affect the accuracy of your measurements.
Techniques that work well in the vacuum environment of a satellite with
no atmospheric seeing effects may be less effective when used at the
bottom of the atmosphere when the stellar image isnt diffraction limited.
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